Chromium Chalcogenide based Thermoelectrics

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For many commercial processes like combustion engines losses due to waste heat are inevitable. Thermoelectric materials mounted in a thermoelectric generator can partially recover the otherwise lost energy by converting the existent temperature gradient to an electrical voltage. However, current efficient thermoelectric materials consist of rare and hazardous elements like Lead, Antimony, Tellurium, etc.The project Chrome chalcogenide based Thermoelectrics investigates binary (CrxCrS2) and ternary (MCrS2 (M = Cu, Ag)) chrome chalcogenides as up until now unexplored thermoelectric materials. Besides the relative ease of accessibility of consisting elements, chrome chalcogenides are known semiconductors which can be highly doped. Hence, substitution of anions and cations is possible for a broad compositional range without alterations to the general structure. The influence of substitution on real structure and thermoelectric properties, especially Seebeck coefficient as well as electrical and thermal conductivity will be thoroughly investigated during this project.The electrical conductivity can be modified by changing the number of charge carriers and the width of the band gap. On the other hand, the Seebeck coefficient conceivably increases due to additional spin entropy. A decrease of the thermal conductivity can be achieved by substitution and topotactic reactions, introducing specific defect structures (point defects, planar defects and nanoprecipitates). To rationalize the chemical, micro- and nanostructural properties of these new thermoelectric materials challenging and partially novel to-be-developed Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD) methods have to be used in order to investigate correlations between synthesis, composition, (micro)structure and thermoelectric properties. The combination of both methods allows a complete and quantitative model for complex materials on all relevant length scales. For the thermoelectric characterization, a new type of measurement set-up will be developed which enables quick and reliable determination of the characteristic variables for thermoelectric materials of numerous samples. Particularly promising materials will be investigated in detail by in and ex situ TEM and XRD heating experiments to investigate structural changes upon cyclic heating; these will be correlated to alterations of thermoelectric properties.Ultimately, this research project also addresses relevant application-technological topics like the preparation of low ohmic contacts for integration in chrome chalcogenide based thermoelectric generators. Therefore, appealing questions arise for basic research, upon answering; a significant benefit for the thermoelectric community can be expected, like long term stability of thermoelectric materials and the specific failure mechanism.